Lemony Asparagus Soup

Bring a large saucepan of water to a boil. Fill a large bowl with ice water. Boil all of the asparagus until bright green and barely tender, about 3 minutes. Drain and transfer to the ice water to cool, then drain again. Cut the 12 asparagus tips in half lengthwise and set aside. Wipe out the saucepan. Add the olive oil and heat until shimmering. Add the sliced onion to the saucepan and cook over moderately high heat, stirring, until softened, about 5 minutes. Add the chicken stock and lemon zest strips, bring to a boil and simmer for 5 minutes. Add the asparagus pieces and cook until tender, about 5 minutes. Discard the lemon zest strips. Working in batches, puree the soup in a blender. Return the soup to the saucepan and stir in the lemon juice. Season with salt and white pepper and reheat if necessary. Ladle the soup into bowls and garnish with the asparagus tips and grated lemon zest. Serve hot or chilled.